Propylene impact copolymers are commonly used in a variety of applications where strength and impact resistance are desired, such as molded and extruded automobile parts, household appliances, luggage and furniture. Propylene homopolymers are often unsuitable for such applications because they tend to be brittle and have low impact resistance particularly at low temperature, whereas propylene impact copolymers are specifically engineered for applications such as these.
A typical propylene impact copolymer contains two phases or components, a polypropylene (typically homopolypropylene) component and a propylene copolymer component. These two components are usually produced in a sequential polymerization process wherein the polypropylene produced in a first reactor is transferred to a second reactor where copolymer is produced and incorporated within the matrix of the homopolymer component. The copolymer component typically has rubbery characteristics and provides the desired impact resistance, whereas the polypropylene component provides overall stiffness.
Propylene impact copolymers are traditionally made using Zeigler Natta catalyst systems. Many process variables influence the resulting impact copolymer and these have been extensively studied and manipulated to obtain various desired effects. Examples of these may be found in U.S. Pat. No. 5,166,268 (describing a “cold forming” process); U.S. Pat. No. 5,258,464 (describing propylene impact copolymers with improved resistance to “stress whitening”) and U.S. Pat. No. 5,250,631 (describing a propylene impact copolymer having a homopolypropylene first component and an ethylene/butene/propylene terpolymer second component).
The composition of the first and second component may also influence the resulting impact copolymer, and any applications using that polymer. For example, U.S. Pat. No. 5,948,839 describes a conventional first component and 25 to 45 weight percent ethylene/propylene second component having from 55 to 65 weight percent ethylene. This impact copolymer composition has a melt flow of from 7 to 60 dg/min, and may be used to produce films useful in articles such as diapers.
U.S. Pat. No. 7,816,452 describes a process for preparing propylene polymer compositions in at least a two stage process, wherein in a first stage propylene homopolymer or a propylene copolymer, containing at least 85% by weight of propylene is prepared and in a second stage ethylene and at least one comonomer selected from propylene and C4 to C10 1-alkenes are polymerized to give an ethylene polymer comprises about 90% by weight of ethylene, and wherein the amount of ethylene polymer in the propylene polymer composition ranges from 10% to 50% by weight and the propylene polymer composition has an MFR (ISO 1133 at 230° C., 2.16 kg) of from 2 to 50 g/10 min. The propylene polymer compositions may be particularly useful for the production of molding, especially by injection or blow molding.
Recently, efforts have been made to prepare propylene impact copolymers using the metallocene catalysis technology in order to capitalize on the inherent benefits such catalysts are known to provide. Metallocene catalyzed homopolymers typically have narrow molecular weight distributions, and low extractables and a variety of other favorable properties associated therewith. Metallocene catalyzed copolymers typically have narrow composition distributions in addition to narrow molecular weight distribution and low extractables. Unfortunately, most known metallocenes are not able to provide copolymer components with high enough molecular weight under commercially relevant process conditions. The resulting propylene impact copolymers therefore tend to have poor impact strength compared to their conventionally catalyzed counterparts.
U.S. Pat. No. 5,990,242 approaches this problem by using an ethylene/butene (or higher α-olefin) copolymer second component, rather than a propylene copolymer, prepared using a hafnocene type metallocene. Such hafnium metallocenes in general are known for producing relatively higher molecular weight polymers, however, their activities are much lower than the more commonly used zirconocenes. In any event, the second component molecular weights and intrinsic viscosities are lower than desired for good impact strength. U.S. Pat. No. 6,492,465 also describes the use of metallocene catalysts to provide propylene impact copolymers.
The present inventors have discovered new propylene compositions having the benefits of metallocene catalyzed polymers in addition to properties needed for high impact strength. Importantly, these polymers can be economically produced using commercial-scale processes.